Alkoxides, polymeric

Figure 2. Relation between the intrinsic viscosity and the number-averaged molecular weight of trimethylsilylated alkoxide polymeric particles for Si(0C2H5) solutions with different r values. (Reproduced with permission from Ref. 3. Copyright 1984 North Holland Physics Publishing.)...

An existing polymer with an appropriate end group can be reacted with an alkoxyamine for instance, alkoxide polymerization of ethylene oxide yields a hydroxyl-terminated polymer that undergoes substitution (in the presence of sodium hydride) with a halogen-containing alkoxyamine. 

Haggard and Lewis have shown that tetrabutylammonium alkoxides polymerize methacrylates in alcohol solution [68]. Block polymer synthesis is not possible since the ester groups on the monomer exchange with the solvent. 

Absence of Termination Processes. The possibility of having carbanionic species that show a negligible rate of termination is now realized. In other words, just as the growing chain end in the alkoxide polymerization of ethylene oxide represents a"stable" salt of an alkali metal and an alcohol, the styryl sodium chain end, in the polymerization of styrene by sodium naphthalene, represents a "stable" salt of sodium and a hydrocarbon. This relationship was first noted in the particular case of the sodium naphthalene systems in which the organometallic species is stabilized by a high degree of solvation by an ether, such as tetrahydrofuran, so that no observable side reactions exist, that is, termination of chains, at least not within the time scale of the polymerization reaction. 

Coordination/Insertion polymerization employed less reactive metal car-boxylates, oxides and alkoxides. Polymerization takes place in the presence of tin, zinc, aluminum and other heavy metal catalysts with tin (II) and zinc yielding the purest polymers. The mechanism is shown in Figure 7.6 [12]. 

Sanchez J. 1994. Kinetics and Models of Silicon Alkoxide Polymerization. [AVM] Cairncross R.A. 1994. Solidification Phenomena During Drying of Sol-to-Gel Coatings. [LEE, LES]... 

Precaution Combustible Incompat. with oxidizers, acids, peroxides, azo compds., metal alkoxides, polymerization initiators avoid excessive heat, ignition sources... 

Precaution Combustible incompat. with oxidizers, adds, peroxides, azo compds., metal alkoxides, polymerization initiators avoid excessive heat, ignition sources Hazardous Decomp. Prods. CO , NO , possibly amnwnia Storage Store in well-ventilated area in tightly closed containers Ken-React NZ 39 [Kenrich Petrochems.]... 

Aluminum alkoxides, polymerizations of poly[bis(methoxyethoxyethoxy)phos-phazene] molecular composites, 258-266 Aluminum nitride applications, 427... 

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

Other Rea.ctions, The photolysis of ketenes results in carbenes. The polymeriza tion of ketenes has been reviewed (49). It can lead to polyesters and polyketones (50). The polymerization of higher ketenes results in polyacetals depending on catalysts and conditions. Catalysts such as sodium alkoxides (polyesters), aluminum tribromide (polyketones), and tertiary amines (polyacetals) are used. Polymers from R2C—C—O may be represented as foUows. 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

In anionic polymerization the reaction is initiated by a strong base, eg, a metal hydride, alkah metal alkoxide, organometaHic compounds, or hydroxides, to form a lactamate ... 

Glycol Titanates. Primary diols (HOGOH), such as ethylene glycol and 1,3-propanediol, react by alkoxide interchange at both ends, yielding insoluble, white soflds that are polymeric in nature (18,61—63) ... 

Besides direct hydrolysis, heterometaHic oxoalkoxides may be produced by ester elimination from a mixture of a metal alkoxide and the acetate of another metal. In addition to their use in the preparation of ceramic materials, bimetallic oxoalkoxides having the general formula (RO) MOM OM(OR) where M is Ti or Al, is a bivalent metal (such as Mn, Co, Ni, and Zn), is 3 or 4, and R is Pr or Bu, are being evaluated as catalysts for polymerization of heterocychc monomers, such as lactones, oxiranes, and epoxides. An excellent review of metal oxoalkoxides has been pubUshed (571). 

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